The gaseous discharge lamp is a light source in which light is produced by gas ionization brought about by an electric discharge. To initiate the discharge, the electrodes of a gaseous discharge lamp must be supplied with a rather high voltage (from hundreds to thousands of volts) capable of breaking through the gap between the electrodes to initiate ionization and gaseous discharge. Until the discharge occurs, a gaseous discharge lamp has a very high impedance, the current in the lamp being practically absent. After initiation of the discharge, current flows through the lamp and its impedance decreases. To prevent damage to the lamp, the current in the ignited lamp must be limited. It is common practice to use for this purpose a reactor connected in series with the lamp. If the lighting system comprises a plurality of gaseous discharge lamps, then each of them is usually connected to the supply source through a separate reactor. A parallel connection of gaseous discharge lamps through a common reactor cannot be tolerated because the initiation of ionization in one lamp leads to reduction in the lamp voltage and thus prevents the firing of the other lamps.
Known in the art is a lighting system comprising an alternating-current voltage source and gaseous discharge lamps each connected to the voltage source through a reactor (cf. a book by O. G. Bulatov, V. S. Ivanov and D. I. Panfilov "Tiristornye Skhemy Vkljucheniya Vysokointensivnykh Istochnikov Sveta", published by "Energiya", Moscow, 1975, page 39, FIG. 2-20).
The voltage which initiates ionization in a gaseous discharge lamp is several times greater than the voltage to be supplied to the lamp after ignition. Therefore the voltage at the reactor is usually 2 to 2.5 times greater than the voltage drop across the ignited lamp so that the reactor should be designed for a relatively great electric power, with the result that it is relatively great in weight and size. This, in turn, leads to significant power losses in the winding and core of the reactor. Besides, the presence of the reactor brings about deterioration in the power factor and thus necessitates the use of power factor compensating capacitors. As a result, the lighting fixtures have great weight and size.
To prevent the reactor from saturation during operation, its core must be provided with an air gap the presence of which, in case the core is improperly assembled, may cause "humming" of the reactor during operation. Therefore, the necessity to assemble a core with an air gap complicates the making of the reactor and thus increases the cost of the lighting system.
The voltage provided by the supply source may prove to be insufficient to fire a gaseous discharge lamp, especially when high-pressure gaseous discharge lamps are used, e.g. when a high-pressure sodium vapour lamp having an ignition voltage of 1 kilovolt is connected to an alternating-current network of 220 or 380 volts. In such cases it is necessary to have additional starting devices, such as thermal relays having their contacts connected across the lamps to provide upon their opening a sharp increase in the lamp voltage due to the e.m.f. of self-induction induced in the reactor (for low-pressure lamps), or special circuits generating pulses of high voltage sufficient to break through the gap between the electrodes (for high-pressure lamps). The need to employ additional starting devices complicates the lighting system. A similar problem arises when several gaseous discharge lamps are connected in series because the breakdown voltage increases approximately in proportion to the number of series-connected lamps.
The voltage applied to gaseous discharge lamps after ignition must not deviate significantly from the nominal value because even a relatively small increase in the voltage with respect to the nominal value leads to a sharp reduction in the service life of the lamp due to quick deterioration of the electrodes, whereas a relatively small reduction in the voltage makes the ignition of the lamp unreliable. The permissible value of deviation in the lamp voltage is usually no more than 5 to 7 percent. Because of this, variations in the output voltage of the alternating-current network supplying gaseous discharge lamps, occurring when the electrical devices connected to the network, including the lamps themselves, are switched on and off, adversely affect the operating reliability of the lighting system.
In case of a lighting system comprising a great number of gaseous discharge lamps consuming a large current from the supply source, substantial energy losses occur in the wires connecting the lamps to each other and to the supply source. The current consumed by the lamps and thus the energy losses may be reduced by providing a higher voltage from the supply source. In the known lighting systems this voltage, for safety reasons, cannot be greatly increased without substantial complication of the lighting system (e.g. by using step-down transformers), which creates relatively great energy losses in the lighting systems having a great number of lamps.
Besides, if in such lighting systems the wires connecting the lamps to each other have a great length, e.g. if the lighting system is intended for lighting streets or motor roads, the voltage in the lighting system, because of the voltage drop in the wires, will drop relatively quickly with increases in the distance from the transformer substation connecting a corresponding section of the lighting system to the power line. Since, as pointed out above, significant deviations of the voltage at a gaseous discharge lamp from the nominal value cannot be tolerated, the length of the section supplied from one substation will be relatively small, which makes it necessary to provide a great number of substations connecting such a lighting system to the power line, as a result of which the construction and maintenance costs are increased.
Also known in the art is a lighting system comprising an alternating-current voltage source and gaseous discharge lamps connected to the voltage source through an autotransformer (cf. U.S. Pat. No. 3,872,350 issued Mar. 18, 1975). In such a lighting system the windings of the autotransformer are magnetically loosely coupled to each other in order to provide a rise in the voltage at the transformer output when the lamps are being switched on, which is necessary to initiate gaseous discharge, and to provide reduction in the lamp voltage after ignition.
The employment of an autotransformer makes it possible to reduce its rated power in comparison with the reactor. However, because of the loose magnetic coupling between the transformer windings, this power remains significantly (about 70 to 80 percent) greater than the power consumed by the lamps connected to the transformer. Therefore, in such a system the lighting fixtures still have relatively great size and weight. The loose magnetic coupling between the windings may be achieved by providing an air gap in the transformer core, which complicates the making of the transformer, or by increasing the length of the magnetic circuit between the core portions at which the transformer windings are wound, which leads to a substantial increase in the transformer size and weight. Besides, the loose coupling between the transformer windings strongly deteriorates the power factor thus necessitating the use of power factor compensating capacitors.
The employment of an autotransformer provides a certain increase in the voltage supplied to the lamps when ionization is initiated, which makes it possible to connect two low-pressure lamps to the secondary winding of one transformer. However, in order to achieve a further increase in the voltage applied to the lamps at ignition, it is necessary to use additional starting devices. Besides, to provide ignition of two lamps by one transformer, one of them must be shunted by a capacitor, which complicates the lighting system.
In the case of a lighting system having a plurality of gaseous discharge lamps connected to the supply source through a plurality of parallel-connected autotransformers, variations in the supply voltage, as in the case of a lighting system with current-limiting reactors, will adversely affect the operating reliability of the lighting system. The energy losses in the wires of the lighting system in such a case will be also relatively great because, for safety reasons, the voltage of the supply source cannot be increased without substantial complication of the lighting system. If in such a lighting system the wires connecting the lamps have a great length, the voltage in the lighting system, as in a lighting system employing current-limiting reactors, will relatively quickly fall with increases in the distance from a transformer substation so that a great number of substations is required and the construction and maintenance costs are increased.
The principal object of the present invention is to provide a lighting system, which should be made so as to reduce its size, weight and cost and to increase at the same time the voltage applied to the gaseous discharge lamps at ignition without the use of additional starting devices, to reduce the current consumed in the lighting system at a rated load without adding complexity to the system, and to eliminate the influence of the resistance of the connecting wires and of load variations on the voltage supplied to each of the lamps after ignition, thereby decreasing weight, size and cost of lighting fixture, increasing reliability of lighting system operation, and reducing energy losses and construction and maintenance costs for lighting systems in which the wires connecting the lamps to one another have a considerably length.